Critical performance parameters of optical spectrum analyzers (OSAS), such as signal selectivity and measurement sensitivity, depend on the characteristics of optical filters within the OSAs. Optical filters having narrow bandwidth, low insertion loss and wide tuning range are advantageous, enabling OSAs to test complex optical signals, such as those within dense wavelength division multiplexed (DWDM) optical telecommunication systems. Presently available OSAs use optical filters having center wavelengths that are tuned by varying the tilt angle of an interference filter relative to the beam of the applied optical signal. However, the filter's center wavelength depends on the polarization state of the applied optical signal and as the tilt angle of the interference filter varies, the filter bandwidth and insertion loss increase, making the OSA unsuitable for measuring optical signals within a DWDM telecommunication system.